The warning came in a new report launched on Wadden Sea Day - a platform for recent research on the marine World Heritage Site that borders the Netherlands, Germany and Denmark.

As well as reducing biodiversity, the abundant alien species could also prove an economic burden to the Wadden Sea region. Costs are incurred through managing or eradicating invasive alien species and the impact on the food chain from non-native wildlife means that fishermen's yields are often reduced.

Grasses, mussels and jellyfish are the most damaging invaders in the Wadden Sea.

The common cord-grass (Spartina) is the most damaging invasive plant as it encourages the build-up of sediment - thus transforming sea's tidal flats into salt marshes. The plant was deliberately introduced into the Wadden Sea to enhance the development of such salt marshes, but eradication efforts failed and the species is now spreading.

Another invader - the Pacific Oyster (Crassostrea gigas) - was introduced from Asia into the mussel cultures of the Wadden Sea in the 1990s. Since then, Pacific oysters have begun to invade native blue mussel beds and create their own oyster reefs throughout the Wadden Sea, causing a food shortage for birds that feed on blue mussels. Although there has been an increase in blue mussel populations in the Dutch parts of the Wadden Sea, numbers in the Danish and German areas have dropped. There are major concerns that the Pacific oyster might displace domestic blue mussel beds.

Invading jellyfish could also be threatening fish populations in the Wadden Sea. The sea walnut (Mnemiopsis leidyi) is native to western Atlantic coastal waters but was first recorded in the Wadden Sea in 2006. It is thought that it arrived via the ballast water of ships - water carried by commercial vessels for stability that is then discharged upon arrival in port.

The sea walnut consumes zooplankton, crustaceans, other jellyfish and the eggs and larvae of fish. Elsewhere, this species is being blamed for the striking decrease of anchovy in the Black and Caspian Seas. Conservationists are concerned that the same phenomenon might occur in the Wadden Sea if the numbers of sea walnut continue to rise.

The invading species are damaging one of Europe's most diverse marine habitats. A 500 kilometre-long complex of mud and sand flats, salt marshes, islands, dunes, estuaries, gullies and open waters, the Wadden Sea is one of the last remaining natural inter-tidal ecosystems in Europe. It supports a huge number of plant and animal species and between 10 and 12 million birds visit the Wadden Sea during their migrations ever year.

"Second only to habitat fragmentation, invasive alien species have become a major driver of biodiversity loss", said Elizabeth Maruma Mrema, Executive Secretary of the Convention on the Conservation of Migratory Species of Wild Animals (CMS) Secretariat, based in Bonn. "CMS is concerned about alien species invading ecosystems at an unprecedented rate. We have started collecting data and encourage further research on the impact on endangered migratory species. Indeed, during the International Year of Biodiversity, efforts to control the introduction of alien species are helping protect the Wadden Sea ecosystem."

The Wadden Sea Agreement (1990) was concluded under CMS and promotes the conservation of seals in the region. The Wadden Sea is also an important habitat for migratory birds, whales and dolphins - all three of which are the subject of other international CMS Agreements.

The 5th Wadden Sea Day was organised by the National Park Administration Wadden Sea Lower Saxony and the Common Wadden Sea Secretariat. At the event, the Wadden Sea countries (the Netherlands, Germany and Denmark) discussed the new challenge posed by alien marine species and how they can be met by trilateral policies

After the entry of the Wadden Sea on the World Heritage List in June 2009, the World Heritage Committee is encouraging the Wadden Sea countries to implement a strict monitoring programme to control invasive species.

The Quality Status Report 2009 suggested developing a Trilateral Management Plan for the entire Wadden Sea to effectively address and deal with invasive alien species, such as specifying trilateral targets and integrating them in the European strategy on invasive alien species.

During the Trilateral Governmental Conference held in March 2010 on the German island of Sylt, ministers stated their concern about the possible impacts on the Wadden Sea from human intervention, invasive alien species and particularly climate change. They agreed to support current international efforts to prevent and manage the introduction of alien species by ratifying the 2004 International Convention for Control and Management of Ships' Ballast Water and Sediments (Ballast Water Management Convention) as soon as possible. In addition, they instructed the Wadden Sea Board to develop a common strategy for dealing with the introduction of alien species in the Wadden Sea by the next Ministerial Conference in 2013.

The scientific conference was a first step towards implementing the agreement reached in the Ministerial Declaration.

Notes to editors:

The Convention on the Conservation of Migratory Species of Wild Animals (CMS), also known as the Bonn Convention, works for the conservation of a wide array of endangered migratory animals worldwide through the negotiation and implementation of agreements and species action plans. It is an intergovernmental treaty, concluded under the aegis of the United Nations Environment Programme (UNEP). At present, 114 countries are parties to the Convention. (www.cms.int)

The Agreement on the Conservation of Seals in the Wadden Sea was the first legally binding agreement concluded under CMS in 1990. The Common Wadden Sea Secretariat contains provisions on research and monitoring, on taking and on the protection of habitats according to the Conservation and Management Plan for the Wadden Sea Seal Population 2007-2010. It collects information on activities that have significant effects on the natural environment of the Wadden Sea and suggests appropriate actions.

The Common Wadden Sea Secretariat and the Trilateral Monitoring and Assessment Group published the "Quality Status Report for the Wadden Sea 2009".